Western Equatorial Atlantic Research Articles

Magnetic petrology of equatorial Atlantic sediments: Electron microscopy results and their implications for environmental magnetic interpretation

The magnetic microparticle and nanoparticle inventories of marine sediments from equatorial Atlantic sites were investigated by scanning and transmission electron microscopy to classify all present detrital and authigenic magnetic mineral species and to investigate their regional distribution, origin, transport, and preservation. This information is used to establish source‐to‐sink relations and to constrain environmental magnetic proxy interpretations for this area. Magnetic extracts were prepared from sediments of three supralysoclinal open ocean gravity cores located at the Ceará Rise (GeoB 1523‐1; 3°49.9′N/41°37.3′W), the Mid‐Atlantic Ridge (GeoB 4313‐2; 4°02.8′N/33°26.3′W), and the Sierra Leone Rise (GeoB 2910‐1; 4°50.7′N/21°03.2′W). Sediments from two depths corresponding to marine isotope stages 4 and 5.5 were processed. This selection represents glacial and interglacial conditions of sedimentation for the western, central, and eastern equatorial Atlantic and avoids interferences from subsurface and anoxic processes. Crystallographic, elemental, morphological, and granulometric data of more than 2000 magnetic particles were collected by scanning and transmission electron microscopy. On basis of these properties, nine particle classes could be defined: detrital magnetite, titanomagnetite (fragmental and euhedral), titanomagnetite‐hemoilmentite intergrowths, silicates with magnetic inclusions, microcrystalline hematite, magnetite spherules, bacterial magnetite, goethite needles, and nanoparticle clusters. Each class can be associated with fluvial, eolian, subaeric, and submarine volcanic, biogenic, or chemogenic sources. Large‐scale sedimentation patterns are delineated as well: detrital magnetite is typical of Amazon discharge, fragmental titanomagnetite is a submarine weathering product of mid‐ocean ridge basalts, and titanomagnetite‐hemoilmenite intergrowths are common magnetic particles in West African dust. This clear regionalization underlines that magnetic petrology is an excellent indicator of source‐to‐sink relations. Hematite encrustations, magnetic spherules, and nanoparticle clusters were found at all investigated sites, while bacterial magnetite and authigenic hematite were only detected at the more oxic western site. At the eastern site, surface pits and crevices were seen on the crystal faces indicating subtle early diagenetic reductive dissolution. It was observed that paleoclimatic signatures of magnetogranulometric parameters such as the ratio of anhysteretic and isothermal remanent magnetizations can be formed either by mixing of multiple sources with separate, relatively narrow grain size ranges (western site) or by variable sorting of a single source with a broad grain size distribution (eastern site). Hematite, goethite, and possibly ferrihydrite nanoparticles occur in all sediment cores studied and have either high‐coercive or superparamagnetic properties depending on their partly ultrafine grain sizes. These two magnetic fractions are generally discussed as separate fractions, but we suggest that they could actually be genetically linked.

IMAGES - The hydrological cycle and ocean temperatures: A paleo-perspective

runoff . Such records would provide information on past changes in the hydrological cycle, in which surface ocean temperature and circulation changes are strongly tied with fl uctuations in precipitation over the continents. PAGES has identifi ed landocean climatic linkages as a major objective for the coming years. As water vapor exerts a strong eff ect on Earth’s climate, improved future scenarios of climate change need to take into account changes in the hydrological cycle more than before. The principle processes of past linkages between ocean circulation changes, ocean-atmosphere water exchange, and shifts in the hydrological regimes over the continents are well understood. However, not much is known about the magnitudes of change, such as variations in the evaporation-precipitation balance between the ocean and adjacent continents or between diff erent ocean basins. Quantitative estimates of this kind could be very helpful in testing the performance of Earth system models with a hydrological cycle and therefore, essential for more accurate climate change predictions. The study of past changes in surface ocean temperatures and salinity is one of IMAGES major challenges. Examples from the equatorial Atlantic using paired Mg/ Ca and δ 18 O measurements on planktonic foraminiferal shells, show that the western and eastern basins have historically experienced very diff erent sea surface temperature and salinity variations, particularly during the Younger Dryas Period and the Holocene (Fig. 1). This can be explained by diff erences in the response to remote and local forcing. The western equatorial Atlantic is strongly infl uenced by the thermohaline circulation via the Brazil Current, whereas the eastern Atlantic is intimately coupled to the West African monsoon moisture transport and continental runoff and thus depends on changes in tropical ocean surface temperatures as well as on shifts of the Intertropical Convergence Zone (ITCZ).

Cloud‐SST feedback in southeastern tropical Atlantic anomalous events

Using satellite‐based cloud measurements for 1984–2004, the interannual variability of the low‐level cloud cover over the tropical Atlantic Ocean in austral winter (June–July–August, JJA) is examined. It is found that the leading pattern of the low‐cloud anomalies in this season is a modulation of the climatological center of the cloud cover over the southeastern tropical Atlantic Ocean off the Angola and Namibia coasts. The fluctuation of cloud amount there occurs on both interannual and longer timescales. The relationship between this low‐cloud anomalous pattern and basinwide ocean‐atmosphere anomalies is studied through a composite analysis based on the objectively selected major low‐cloud deficit and excess years. For the composites we intentionally use data sets mainly based on satellite measurements for the past one to two decades to minimize the potential influences of the bias in the model‐based ocean‐atmosphere analyses. The composites show that the JJA anomalous cloud pattern is strongly influenced by the sea surface temperature (SST) anomalies of the equatorial and southeastern tropical Atlantic Ocean in the previous summer. The anomalous surface warm events off the southwestern coast of Africa near 15°S in January and February are usually initiated dynamically by remote forcing from the westerly wind anomalies over the western equatorial Atlantic, preceding the cloud anomalies. During the next few months, the warm water is spread into the southeastern tropical Atlantic Ocean and is conducive to deficit low‐cloud cover in subsequent JJA. The reduced low‐cloud cover in turn forces positive SST tendency in a larger area of the southeastern Atlantic Ocean by changing the amount of the local solar radiation reaching the sea surface. In June and July, this process moves the major center of the SST anomalies away from the coast and closer to the equator when the coastal process weakens. The low cloud‐radiation‐SST feedback also plays a role in the slow westward expansion of the SST anomalies in late austral winter and spring. Overall, the influence of the cloud fluctuation is an important component in the evolution of the southeastern tropical Atlantic anomalous events.

Report on the collision between a spinner dolphin and a boat in the Fernando de Noronha Archipelago, Western Equatorial Atlantic, Brazil

This paper reports a case of collision between an individual spinner dolphin and a boat in the Fernando de Noronha Archipelago, as an example of disturbance potentially caused by tourism industry activities. Photos illustrating the inflicted injuries on the animal are presented, and preventive measures to avoid this kind of accidents are proposed. This is the first report on a collision between spinner dolphins and boats.

Mid-Cretaceous (Albian–Santonian) sea surface temperature record of the tropical Atlantic Ocean

Paleoclimate records of geologic time periods characterized by extreme global warmth such as the mid-Cretaceous are important for a better understanding of the Earth9s climate system operating in an exceptionally warm mode. Here we applied an organic geochemical proxy (TEX 86 ) on organic matter–rich Albian–Santonian sediments, recovered from Ocean Drilling Program Leg 207 Sites 1258 and 1259 on Demerara Rise, to reconstruct sea surface temperatures (SSTs) in the western equatorial Atlantic. Preceded by a stepwise Cenomanian warming trend (∼31–35 °C), the onset of the Cretaceous thermal maximum coincided here with the Cenomanian-Turonian boundary event. Once established, this extreme warm climate regime, characterized by averaged tropical SSTs close to 35 °C, lasted up to the Turonian-Coniacian transition. Two pronounced cooler intervals (∼2–3 °C) interrupt this otherwise remarkably stable record, providing the first δ 18 O independent evidence for middle Turonian cooling that previously has been attributed to glacioeustatic sea-level lowering. Coniacian SSTs decline stepwise, reaching a minimum in the Santonian (∼32–33 °C), where cooling is most pronounced, presumably concomitant with the first progressive opening of a deep-water passage through the equatorial Atlantic gateway.

Surface-water chemistry and fertility variations in the tropical Atlantic across the Paleocene/Eocene Thermal Maximum as evidenced by calcareous nannoplankton from ODP Leg 207, Hole 1259B

Calcareous nannoplankton assemblages at Ocean Drilling Program (ODP) Site 1259 on Demerara Rise (western equatorial Atlantic) underwent an abrupt and fundamental turnover across the Paleocene/Eocene Thermal Maximum (PETM) ~55.5 m.y. ago. The PETM is marked by a dissolution interval barren or nearly-barren of nannofossils due to the rapid acidification of the world oceans. Toweius, Fasciculithus, and Chiasmolithus sharply decrease at the onset, whereas Chiasmolithus, Markalius cf. M. apertus, and Neochiasmolithus thrive immediately after the event, which also signals the successive first appearances of Discoaster araneus, Rhomboaster, and Tribrachiatus. The environmental indications of these changes were further investigated by correspondence analysis on quantitative nannofossil counts. The PETM event has been attributed to CO 2-forced greenhouse effects. At Site 1259, the elevated pCO 2 and subsequent lowered surface-water pH values at the onset of the PETM caused intensive carbonate dissolution, producing the nannofossil-barren interval. The chemically stressed habitats may well have also induced the evolution of ephemeral nannofossil “excursion taxa”, such as Rhomboaster and malformed discoasters ( D. araneus and Discoaster anartios). Based on its sudden increase, Markalius cf. M. apertus is considered to have been a local opportunistic species that took advantage of the surface-water changes. At the same time, a presumably higher runoff from continental areas fertilized the western equatorial Atlantic as indicated by an increase in the abundance of r-mode specialists preferring high-nutrient conditions, such as Chiasmolithus, Coccolithus pelagicus, and Hornibrookina arca. Contrasts between the results of this study and previous work at ODP Site 690 in the Southern Ocean, the New Jersey continental margin, and the central paleoequatorial Pacific further demonstrate that the response to the PETM can be influenced by local differences in geologic setting and oceanographic conditions.

Glacial deep water carbonate chemistry inferred from foraminiferal Mg/Ca: A case study from the western tropical Atlantic

We explore using changes in planktonic foraminiferal Mg/Ca with water depth in a single region to infer relative changes in deep water carbonate chemistry. The Mg/Ca ratio in foraminiferal calcite is quantitatively lowered by dissolution. We exploit this dissolution effect to generate a profile of carbonate ion concentration (CO32−) for the deep waters of the tropical Atlantic during the Last Glacial Maximum. We generated Mg/Ca data for three species, G. ruber, G. sacculifer, and N. dutertrei, from three cores located along a depth transect (2.8 to 4.0 km) on the Ceara Rise in the western equatorial Atlantic. We attribute lower Mg/Ca in glacial intervals from the shallow core to changes in sea surface temperature (SST). The even lower glacial Mg/Ca values in the deeper cores are attributed to dissolution and are used to reconstruct a paleo‐CO32− profile. Our down core Mg/Ca data for G. ruber and G. sacculifer confirm that these species are more resistant to Mg loss due to dissolution than N. dutertrei. The greater susceptibility of N. dutertrei's Mg content to dissolution limits the utility of this species in thermocline temperature reconstructions to cores above the lysocline but, significant to this study, makes N. dutertrei useful in quantifying changes in deep water CO32−. Our paleo‐CO32− reconstruction based on N. dutertrei suggests the Last Glacial Maximum CO32− gradient in the Atlantic was much steeper than today, implying that the boundary between deepwater masses in the Atlantic was shallower during the Last Glacial Maximum. Our results suggest comparison of Mg/Ca from multiple cores from one region could similarly provide constraints on changes in carbonate chemistry in other basins and other time periods.

On the origin of the tropical Atlantic decadal oscillation based on the analysis of the ICOADS

AbstractFrom an analysis of the International Comprehensive Ocean–Atmosphere Data Set, we were able to confirm salient features of the tropical Atlantic decadal oscillation illustrated in previous works. These features include antisymmetric behaviour of surface air pressure and sea surface temperature with two centres of action residing in the tropical North and South Atlantic Oceans. In addition to the recognized features of the oscillation, we found a new aspect: the principal mechanism regulating the oscillation differs between the north and south centres of action. Decadal components of the relative humidity time series show a maximum value of lag cross‐correlation at a lag of −2 years. Such a temporal evolution was irrespective of the centres of action and the reference time series. The two reference series employed are the sunspot numbers and the southerlies over the western equatorial Atlantic. Associated with this, statistically significant phase shifts between the decadal components of variables exist, especially in the tropical North Atlantic Ocean. For the decadal oscillation, relative humidity change was dominated by specific humidity change over the tropical North Atlantic and by air temperature change over the tropical South Atlantic. For the frequency band of the solar cycle, the variation of relative humidity seemed to amplify the Earth's response to the sun's radiation changes that had previously been considered too small to affect the Earth's climate. By incorporating our new findings with the known features of the oscillation, we can suggest that the variability of solar radiation may be crucial as an originator, or at least as a regulator, of the oscillation in combination with the climate distribution of clouds and water vapour over the tropical Atlantic. Copyright © 2006 Royal Meteorological Society.

Seasonal changes in the mixed and barrier layers in the western Equatorial Atlantic

Climate is closely related to the dynamics of the surface layer of the tropical Atlantic and the exchange between this latter and the atmosphere, and wearther forecasting will improve with increasing understanding of the processes that govern the relative distribution of thermodynamic properties of the water column. This paper focuses on the isolation of warm surface waters from the cold ones of the deep ocean by a salinity induced barrier layer (BL) in the western equatorial Atlantic (3ºS-7ºN; 40º-52ºW), based on 487 CTD profiles (REVIZEE - 1995-2001). The main process contributing to the seasonal BL formation is the discharge of low salinity waters from the Amazon river. During boreal late winter/spring (Mar-May; high river discharge), deeper isothermal (Z T) and mixed layers (Z M) prevail and the formation of a 16m-thick BL was clearly determined the formation of a salt-induced marked pycnocline within a deeper isothermal layer. However, during the boreal autumn (Oct-Dec; low river discharge), density stratification was mainly determined by temperature distribution (Z M m Z T; BLT = Z M - Z T m 0). There was no clear register of a BL on the Amazon shelf, but a maximum BL (40 m) formed near the shelf break at 45°W.

Stable isotopes in precipitation recording South American summer monsoon and ENSO variability: observations and model results

The South American Summer Monsoon (SASM) is a prominent feature of summertime climate over South America and has been identified in a number of paleoclimatic records from across the continent, including records based on stable isotopes. The relationship between the stable isotopic composition of precipitation and interannual variations in monsoon strength, however, has received little attention so far. Here we investigate how variations in the intensity of the SASM influence δ18O in precipitation based on both observational data and Atmospheric General Circulation Model (AGCM) simulations. An index of vertical wind shear over the SASM entrance (low level) and exit (upper level) region over the western equatorial Atlantic is used to define interannual variations in summer monsoon strength. This index is closely correlated with variations in deep convection over tropical and subtropical South America during the mature stage of the SASM. Observational data from the International Atomic Energy Agency-Global Network of Isotopes in Precipitation (IAEA-GNIP) and from tropical ice cores show a significant negative association between δ18O and SASM strength over the Amazon basin, SE South America and the central Andes. The more depleted stable isotopic values during intense monsoon seasons are consistent with the so-called ’‘amount effect‘’, often observed in tropical regions. In many locations, however, our results indicate that the moisture transport history and the degree of rainout upstream may be more important factors explaining interannual variations in δ18O. In many locations the stable isotopic composition is closely related to El Niño-Southern Oscillation (ENSO), even though the moisture source is located over the tropical Atlantic and precipitation is the result of the southward expansion and intensification of the SASM during austral summer. ENSO induces significant atmospheric circulation anomalies over tropical South America, which affect both SASM precipitation and δ18O variability. Therefore many regions show a weakened relationship between SASM and δ18O, once the SASM signal is decomposed into its ENSO-, and non-ENSO-related variance.

Variação sazonal da estrutura de massas de água na plataforma continental do Amazonas e área oceânica adjacente

Este trabalho analisa as informacoes de temperatura e salinidade obtidas a partir de tres cruzeiros oceanograficos realizados na area da Plataforma Continental do Amazonas e regiao oceânica adjacente. Os dados foram coletados em diferentes periodos do ano, correspondendo a distintas situacoes de aporte de aguas continentais: minima descarga do rio Amazonas (outubro/1997), maxima descarga fluvial (maio/1999), e durante o periodo de transicao (maxima para minima) descarga do rio (agosto/2001). Os resultados permitiram descrever a variacao sazonal da distribuicao tridimensional das quatro massas de agua encontradas: Agua Costeira (AC), Agua Tropical (AT), Agua Central do Atlântico Sul (ACAS) e Agua Intermediaria da Antartica (AIA). Os resultados sugerem que o cisalhamento eolico age sobre a distribuicao temporal das aguas superficiais (AC e AT), enquanto que a distribuicao das massas d'agua de subsuperficie (ACAS e AIA) e influenciada pela variabilidade sazonal da Corrente Norte do Brasil (CNB) e pela presenca de vortices anticiclonicos na regiao de retroflexao da CNB. Os transectos verticais indicam que a ACAS sofre uma expansao de cerca de 70 m (49oW) e de 220m (50oW) durante o periodo de transicao das descargas fluviais, que corresponde a epoca de maxima acao da CNB e da Contra-corrente Norte Equatorial (CCNE).

Annual cycle and interannual variability of the mid-depth tropical Atlantic Ocean

Several shipboard sections acquired in the western equatorial Atlantic along 35 ∘ W allow for the first time to analyze the annual cycle of the density and velocity field in the upper 2000 m within this region. The amplitude of the annual harmonic of the velocity field shows several distinct maxima at the equator. Strong amplitudes up to 15 cm s - 1 are found in the depth range of the equatorial intermediate current (EIC) between 400 and 1000 m depth that are slightly shifted northward with respect to the equator. The meridional structure of the annual harmonics as well as upward phase propagation is consistent with downward propagating odd meridional mode Rossby waves. The observations are compared with a regional numerical model with very high vertical resolution. Good agreement is found between the simulated and observed structure and the amplitude of the annual harmonics. The model results suggest the presence of equatorial beams composed of wind-generated Kelvin and Rossby beams (causing seasonality in the near surface layer) as well as Rossby beams generated by the reflection of Kelvin beams at the eastern boundary (causing seasonality in the depth range of the EIC at the 35 ∘ W section). The annual cycle of the sea surface height (SSH) observed by TOPEX/Poseidon altimetry indicates an east–west seesaw pattern that corresponds to the fast response (about 30 days lag) of the zonal SSH gradient to the annual cycle of the zonal wind field at the equator due to the propagation of lowest baroclinic mode equatorial waves. The increase of the easterly winds in the western equatorial Atlantic associated with El Nino in the beginning of 1997 led almost instantaneously to an adjustment of the zonal SSH gradient with elevated (depressed) SSH in the western (eastern) basin. In contrast, the velocity field at intermediate depths lags the zonal wind forcing by several months up to a year as the model simulation reveals.

Evolution of Interannual Warm and Cold Events in the Southeast Atlantic Ocean

Extreme warm episodes in the southeast Atlantic Ocean, known as Benguela Niños, have devastating environmental impacts and have been shown to be remotely forced. To place these extreme events into perspective, the investigation is here extended to minor warm events as well as to cold episodes. To this end, different sets of observations have been combined with outputs from a numerical simulation of the tropical Atlantic for the period 1982–99. It is shown that both warm and cold surface events develop regularly in the same specific region along the coast of Angola and Namibia. Some cold events compete in magnitude with major warm episodes. Local sea–air heat flux exchanges do not seem to precondition the sea surface in the Angola–Benguela region prior to the arrival of an event. Most warm and cold episodes are large-scale events despite their limited surface signature. They appear to be generated by wind anomalies in the western and central equatorial Atlantic in the same way as Benguela Niños. Seasonal fluctuations of the depth and shape of the tropical thermocline seem partly to control the way subsurface anomalies eventually impact the surface. During austral summer, surface anomalies create an identifiable pool centered near 15°S, whereas in winter they show an elongated pattern along the coast stretching toward the equator. Local upwelling or downwelling favorable wind regimes, as well as local net heat fluxes, may modulate the surface expression of events.

Glacial–interglacial variability in lower North Atlantic deep water: inference from silt grain-size analysis and carbonate preservation in the western equatorial Atlantic

Grain-size records of the terrigenous and calcareous silt fraction, preservation of planktic foraminifera, and benthic foraminiferal stable-isotope data (δ13C, δ18O values of C. wuellerstorfi) at ODP Site 927 on the Ceará Rise (5°27.7′N, 44°28.8′W), are used to reconstruct variations in the history of bottom current strength, ventilation, and carbonate corrosiveness of deep waters during the time interval from 0.8 to 0.3 Ma. Glacial periods are characterized by generally smaller mean sizes of the terrigenous sortable silt fraction (mean(SS)), lower δ13C values, and poorer preservation of planktic foraminifera compared to interglacials. This indicates lower bottom current speeds, larger nutrient contents and more corrosive deep water. By contrast, larger mean(SS) sizes, higher δ13C values, and well preserved planktic foraminifera indicate strong circulation and a well ventilated deep-water mass during interglacials. The observed changes are most likely related to the weakening and strengthening of circulation of Lower North Atlantic Deep Water (LNADW). Cross-spectral analysis between the mean(SS) and benthic δ18O records reveals that minima in mean(SS) occur about 7.6 k.y. after the maximum in ice volume. This indicates a considerable lag time between ice-shield induced changes in LNADW production and subsequent changes in the velocity of LNADW flow in the western equatorial Atlantic. Striking changes in bottom current speed occur regularly during glacial to interglacial transitions. Extremely fine mean(SS) minima point to an almost complete shutdown of bottom current vigor in response to a cessation of LNADW production caused by an enhanced melt water release during the initial phases of deglaciation. However, each of the fine minima extremes is followed by a rapid shift to very high mean(SS) values that indicate strong bottom currents, and hence, vigorous LNADW flow during the early interglacials. After the onset of glacial Stage 12, generally poorer carbonate preservation and higher variability is registered. This coincides with a global decrease in carbonate preservation during the mid-Brunhes (mid-Brunhes dissolution event). Detailed grain-size analysis of the calcareous fine fraction (<63 μm) revealed a considerable reduction of particles in the fraction from 7 to 63 μm during periods of enhanced dissolution. This indicates a preferential dissolution of larger planktic foraminiferal fragments which leads to an enrichment of coccoliths in the calcareous fine fraction.

Variability of silt grain size and planktonic foraminiferal preservation in Plio/Pleistocene sediments from the western equatorial Atlantic and Caribbean

Records of mean sortable silt and planktonic foraminiferal preservation from the Ceará Rise (western equatorial Atlantic) and from the Caribbean are presented to analyze the Pliocene (3.5–2.2 Ma) to Pleistocene (1.6–0.3 Ma) evolution of near-bottom current strength and the carbonate corrosiveness of deep water. During the mid-Pleistocene climate transition (∼1 Ma) a drastic decrease in glacial bottom current strength and an increase in carbonate corrosiveness is registered, demonstrating a substantial decrease in the glacial contribution of the Lower North Atlantic Deep Water (LNADW) to the Atlantic Ocean. Also, an increased sensitivity to eccentricity orbital forcing is registered after the MPT. By contrast, carbonate preservation increases considerably in the deep Caribbean in response to a strong and persistent stable contribution of Upper North Atlantic Deep Water (UNADW). We found evidence for the strongest and most stable circulation within the LNADW cell during the Northern Hemisphere cooling period between ∼3.2 and 2.75 Ma. This is in agreement with the ‘superconveyor model’ which postulates that the highest NADW production took place prior to ∼2.7 Ma. A considerable decrease in bottom current strength and planktonic foraminiferal preservation is observed synchronous with the first occurrence of large-scale continental ice sheets in the Northern Hemisphere. This documents the final termination of the ‘superconveyor’ at ca. 2.75 Ma. However, our data do not support a ‘superconveyor’ in the interval between 3.5 and 3.2 Ma when high-amplitude fluctuations in bottom current flow and preservation in planktonic foraminifera are observed. Because of the great sensitivity of NADW production to changes in surface water salinity, we assume that the high-amplitude fluctuations of LNADW circulation prior to ∼3.2 Ma are linked to changes in the Atlantic salinity budget. After 2.75 Ma they are primarily controlled by ice-sheet forcing. In contrast to the stepwise deterioration of planktonic foraminiferal preservation in the western deep Atlantic, a trend toward better preservation from the Pliocene to Pleistocene is observed in the deep Caribbean. This indicates a long-term increase in the contribution of UNADW to the Atlantic Ocean.

Paleoproductivity, nutrient burial, climate change and the carbon cycle in the western equatorial Atlantic across the Eocene/Oligocene boundary

Paleoproductivity, nutrient burial, and carbon cycling were investigated across the Eocene/Oligocene (E/O) boundary (begin to end; 36.9–32.7 Ma at ∼40 kyr resolution, timescale of Shackleton et al. [1999]) at Ocean Drilling Program Site 925 on the Ceara Rise in the western equatorial Atlantic (3040 m present water depth; 748.26–850.70 mbsf). Downcore bulk sediment records of biogenic barium, total reactive phosphorus, biogenic silica, and calcium carbonate are interpreted to represent export production, net nutrient burial, biogenic opal production, and inorganic carbon burial, respectively. The global positive excursion in δ13C subsequent to the E/O boundary is recorded at Site 925. Export production appears to have been externally forced by orbital parameters at eccentricity frequencies during the study interval, based on spectral analysis of the biogenic barium and reactive phosphorus records. Biogenic silica production or preservation increased after the Eocene/Oligocene boundary to a higher baseline, although overall productivity and nutrient burial did not increase, based on barium and reactive phosphorus records. Thus, although absolute production did not increase at this site, a shift in relative abundance of siliceous versus carbonate productivity may have resulted in a change in relative organic carbon burial. This may have contributed to the positive excursion in global oceanic δ13C subsequent to the Eocene/Oligocene boundary, although the silica maximum persists after the carbon isotope excursion ends.

CaCO3 dissolution in the deep sea: Paced by insolation cycles

Three CaCO3 dissolution events have been documented in the western equatorial Atlantic, one associated with marine isotope stage 5d, one with stage 5b, and one with early stage 4. The intensity of these events appears to require a greatly expanded invasion of waters with a low carbonate ion concentration akin to that characterizing today's deep Pacific (and also the Antarctic Bottom Water). While evident in a core from a depth of 3.4 km, these three events appear to be much less intense in the western equatorial Pacific than in the western Atlantic. The Atlantic results appear to be telling us that the density difference between Southern Ocean and North Atlantic deep waters has been modulated by insolation changes associated with the 23,000‐year precession cycle.

Pseudo dissolution of marine calcite

An enigma exists regarding observations aimed at determining the extent of calcite dissolution on the Ceara Rise in the western equatorial Atlantic. Based on pore-water gradients for Ca, alkalinity, pH, 13C and 14C, a strong case can be constructed that at a depth of 3.3 km about 30% of the calcite rain dissolves [Martin and Sayles, Geochim. Cosmochim. Acta 60 (1996) 243–263; Hales and Emerson, Geochim. Cosmochim. Acta 61 (1997) 501–514; Martin et al., Geochim. Cosmochim. Acta 61 (2000) 1391–1404]. However, benthic chambers deployed at this site record no Ca or alkalinity flux [Jahnke and Jahnke, Geochim. Cosmochim. Acta (2002) in press]. Further, there is no evidence for shell weight loss or shell breakup between 2.8 and 4.1 km water depth on the Ceara Rise [Broecker and Clark, Global Geochem. Cycles (2002) in press]. A possible scenario which accounts for all but the 14C observations [Martin et al., Geochim. Cosmochim. Acta 61 (2000) 1391–1404] is that coatings formed on the calcite are stirred downward into the respiration-CO 2-rich core-top mixed layer and there redissolve. We term this phenomenon pseudo dissolution.

A major dissolution event at the close of MIS 5e in the western equatorial Atlantic

Measurements of the CaCO3 content and the CaCO3 size index on a core from the equatorial Atlantic's Ceara Rise reveal a pronounced dissolution event associated with the transition between MIS 5e and 5d. We tentatively correlate this event with the dramatic drop in atmospheric CO2 content that occurred at the end of MIS 5e. Attempts to quantify the carbonate ion concentration drop associated with this event failed because the shells in this zone had been largely destroyed by dissolution. By leaving out the sonification step in our cleaning procedure, we were able to “rescue” enough shells to weigh, but they showed no dramatic weight loss. We attribute this to the survival of the hardiest shells.

A dramatic Atlantic dissolution event at the onset of the last glaciation

Measurements of the CaCO3 content of the CaCO3 size index and of foraminifera weights demonstrate that a prominent dissolution event occurred in the western equatorial Atlantic at the onset of the last glacial period. The decrease in CO3= ion concentration appears to have been too large to be accounted for by the ocean acidification associated with the demise of excess interglacial biomass. Rather, it appears to have been the result of a temporary strengthening of the penetration of the glacial equivalent Antarctic Bottom Water into the Atlantic. The co‐occurrence of massive Ethmodiscus rex deposition in the eastern equatorial Atlantic bears witness to a reorganization of circulation in the Atlantic. This reorganization may well reflect a temporary collapse of conveyor circulation at the onset of the last glaciation.